Recently the Xenon-1T experiment [1]}
has analyzed events in the low-energy region of 1–30 keV of electron recoil energy
with an exposure of 0.65 tonne-years, while claiming a low background rate of \(76 \pm 2_{\,\mathrm {stat}}\)  events/(tonne-year-keV). The experiment observed an excess of recoil electrons over the background in the 2\(-\) 3 keV range.
The collaboration analyzed the axion couplings to electrons, photons and nucleons, and
the neutrino magnetic moment as possible sources for the signal. However, these models
appear to be in strong tension with stellar constraints [2]}, [3]}, [4]}, [5]}. Another possible source of this excess
is traces of tritium in xenon of size \((6.2 \pm 2.0) \times 10^{-25}\)  mol/mol. The experiment
currently can neither confirm nor exclude such a possibility.
Since the publication of the Xenon-1T results, a variety of models have been proposed which
include light sterile neutrinos [6]}, [7]}, [8]}, [9]},
a goldstino [10]}, an inflaton [11]},
string-motivated models [12]}, [13]}, boosted dark matter [14]}, [15]}, [16]},
and a variety of other models [17]}, [18]}, [19]}, [20]}, [21]}, [22]}, [23]}, [24]}, [25]}, [26]}, [27]}, [28]}, [29]}, [30]}, [31]}, [32]}, [33]}, [34]}, [35]}, [36]}, [37]}, [38]}, [39]}, [40]}, [41]}.
